How does `__bytes__` work for NumPy arrays?

NumPy arrays support __bytes__, which is really great. Though after spending a bit looking at the code, it is not clear how this works. Would someone be able to provide some insight on how this works? Thanks in advance 🙂

Reproducing code example:

In [1]: import numpy as np                                                      

In [2]: a = np.arange(6, dtype="u1").reshape((2, 3), order="F"); a              
Out[2]: 
array([[0, 2, 4],
       [1, 3, 5]], dtype=uint8)

In [3]: a.tobytes()                                                             
Out[3]: b'\x00\x02\x04\x01\x03\x05'

In [4]: bytes(a)                                                                
Out[4]: b'\x00\x02\x04\x01\x03\x05'

Error message:

Nothing! It works great 😄

Numpy/Python version information:

1.18.1 3.6.7 | packaged by conda-forge | (default, Nov  6 2019, 16:03:31) 
[GCC Clang 9.0.0 (tags/RELEASE_900/final)]

[mattip]

Closing, I think the question was answered. Please re-open if there is a need.

[eric-wieser]

Note that your test is misleading, (1) is not the tuple (1,) but just 1. It turns out not to matter though.

What you’re seeing is that the bytes(int) constructor takes precedence over the bytes(buffer) constructor. More specifically, if operator.index(i) works, then memoryview(i) is never tried

If you do bytes(np.int32(4)), you get four zeros. If you do bytes(np.float32(4)), you get the bytes representation of the float.

Unfortunately there’s nothing numpy can do about this. It might be worth suggesting a patch to CPython, changing the semantics from:

def bytes(x):
    try:
        x_int = operator.index(x)
    except TypeError:
        pass
    else:
        return bytes_of_n_zeros(x_int)

    try:
        x_buffer = memoryview(x)
    except TypeError:
        pass
    else:
        return bytes_from_buffer(x_buffer)

    raise TypeError

to

def bytes(x):
    try:
        x_int = operator.index(x)
    except TypeError:
        x_int = None
    try:
        x_buffer = memoryview(x)
    except TypeError:
        x_buffer = None
 
    if x_int is not None and x_buffer is not None:
        warnings.warn("bytes(x) is ambigious - use `bytes(operator.index(x))` or `bytes(memoryview(x))`)
        return bytes_of_n_zeros(x_int)
    elif x_int is not None:
        return bytes_of_n_zeros(x_int)
    elif x_buffer is not None:
        return bytes_of_n_zeros(x_buffer)
    else:
        raise TypeError